1. Field of the Invention
The present invention relates to a folding device mounted on or operatively connected to a copier, printer or similar image forming apparatus for folding a sheet or recording medium or a sheet stack carrying images thereon or a sheet finisher for folding, sorting, stacking, stapling, center-stapling or otherwise finishing the sheet or the sheet stack, and an image forming system consisting of the sheet finisher and image forming apparatus.
2. Description of the Background Art
A sheet finisher positioned at the downstream side of an image forming apparatus for stapling or otherwise finishing a sheet stack is well known in the art. To meet the increasing demand for multiple functions, a sheet finisher having a center-stapling capability in addition to the conventional edge-stapling capability has recently been proposed. Further, a sheet finisher with a center-folding capability in addition to the center-stapling capability has been proposed to fold a center-stapled sheet stack at the center for thereby producing a pamphlet.
A sheet finisher with the binding capability mentioned above uses, in many cases, one or more pairs of fold rollers to fold a sheet stack. In this type of sheet finisher, a flat fold plate is caused to contact the stapled position of a sheet stack and push it into the nip of each fold roller pair, thereby folding the sheet stack.
When the fold plate is used to push a sheet stack into the nip of each fold roller, it is necessary to locate the sheet stack at a position where it faces the fold roller. Therefore, the fold roller pair located at the first stage is exposed to a sheet conveyance path, so that the sheet stack must be conveyed via the position where the fold roller pair is exposed. At this instant, if the sheet stack is relatively thick, then it is likely that the leading edge of the sheet stack facing the fold roller pair is caused to abut against the rollers or to be caught by the rollers and bent thereby.
In light of the above, it has been customary to use means for preventing a sheet stack from contacting the rollers, e.g., a shutter. The shutter prevents the leading edge of a sheet stack from contacting the rollers until it reaches a preselected position. However, the shutter or similar movable member must be driven by a mechanism arranged in the vicinity of the conveyance path, making the sheet finisher bulky. Moreover, the shutter slides on the surface of a sheet when operated, lowering the quality of an image printed on the sheet.
On the other hand, when a sheet stack is relatively thick, the folding device of the type described is apt to fail to sharply fold the sheet stack, leaving a swell in the sheet stack. To solve this problem, Japanese Patent Laid-Open Publication No. 9-2735, for example, discloses a folding system configured to pass a relatively thick, center-folded sheet stack through the nip of a fold roller pair, reverse the rotation of the fold roller pair to again pass the sheet stack through the above nip, and repeat such a procedure a plurality of times. This system, however, has a drawback that the sheet stack, passed through the nip of the fold roller pair a plurality of times, is smeared around the fold due to sliding contact with the fold roller pair, failing to achieve high quality when implemented as a pamphlet.
To protect a sheet stack from smearing mentioned above, Japanese Patent Laid-Open Publication No. 10-218483, for example, proposes a system that lowers a speed at which a sheet stack is pulled out at the time of reversal of rotation of the fold roller pair, thereby efficiently obviating the swell of the sheet stack. This system, however, cannot fully free a sheet stack from smears although reducing them.
Japanese Patent Laid-Open Publication Nos. 2000-72320 and 2001-146363 each teach a system in which two fold roller pairs are arranged such that the former fold roller pair folds a sheet stack, and then the latter fold roller pair makes the fold of the sheet stack more firm. Although this kind of scheme almost frees a sheet stack form smears, it cannot sharply fold a relatively thick sheet stack and therefore fails to solve the problem of swell. Further, the system is not satisfactory as to productivity and whether or not a desired degree of fold can be formed.
Of course, for a given degree of pressure, the fold of a sheet stack becomes dull as the number or sheets constituting the sheet stack increases. In light of this, Japanese Patent Laid-Open Publication No. 3,254,363, for example, proposes a system including selecting means for selecting either one of a first and a second mode and counting means for counting sheets constituting a single sheet stack. In the first mode, a fold roller pair is rotated only in the forward direction to fold a sheet stack one time while, in the second mode, it is rotated in the forward direction and then in the reverse direction to fold the sheet stack two times. The second mode is selected in accordance with the output of the counting means, thereby sharpening the fold of the sheet stack when the sheet stack has more than a preselected number of sheets.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 9-183568 and 2000-198613.